The present invention relates to a method of operating an ICR spectrometer comprising a measuring cell having a plurality of side walls designed as rf electrodes and arranged symmetrically to an axis extending in parallel to the field direction of a homogeneous magnetic field, and further electrically insulated trapping electrodes arranged on both sides of the cell, viewed in the direction of the axis, which trapping electrodes can be supplied with trapping potentials of the polarity of the ions under examination in order to prevent, to a large extent, the ions from leaving the measuring cell in the direction of the axis.
Such an ICR method has been described already by the paper "IonenCyclotronresonanz zur Untersuchung von Ion-Molekul-Reaktionen (the use of ion cyclotron resonance in the examination of ion molecule reactions)" by Lebert, messtechnik, 6 (1970) 109-115. It is particularly well suited for mass analysis of charged particles. For purposes of this method, the ions to be examined are either produced externally and then shot into the ICR measuring cell by means of an ion lens system, or generated in the cell by impact ionization of the residual gas particles by means of an electron beam directed into the ICR cell. The field lines of a homogeneous magnetic field B extend through the inner space of the cell in parallel to the latter's longitudinal axis. Consequently, the particles charged q times with the velocity v, are subjected to the Lorentz force EQU K=qv.times.B.
This force does not obstruct the movement of an ion parallel to the magnetic field lines. If, however, the ion of the mass m exhibits a velocity component v.sub.t perpendicular to the magnetic field, then it is forced by the Lorentz force to move along an orbit whose radius is determined by the balance between the centrifugal force and the Lorentz force: EQU r=mv.sub.t /qB.
The ion moves along this orbit at the cyclotron frequency EQU .omega.=qB/m.
Consequently, any given cyclotron frequency .omega. is assigned to ions of the same mass so that a mass analysis of the ion shower can be performed by frequency analysis. The cyclotron resonance of the ions is produced by resonance excitation of their characteristic movement in the homogeneous magnetic field, through an electric radio frequency (rf) field applied perpendicularly to the magnetic field.
It has been a common problem heretofore of all known ICR cells having a cubic, cylindrical or hyperbolic geometry that due to the existence of finite electric rf field components in axial direction the ions do not only gain the desired energy in a direction radial to the direction of the magnetic field, during the phase of excitation by the electric rf field, but are also accelerated in axial direction. Given the fact that the potential barrier in the axial direction is as low as approx. 1eV, the ions, which have been accelerated in the axial direction, may easily escape from the cell so that they will be lost for the experiment. This problem has been known before and has been described in literature (for example by Kofel et al. in Int. J. Mass Spectrom. Ion Processes 74 (1986) 1-12).
A theoretical solution to this problem would consist in giving the cell an infinite length in the axial direction as the electric rf field would have no axial components in such a cell and would not, therefore, deflect the ions in the axial direction. However, trials with oblong cells with axial dimensions much greater than the diameter of the cell have led to unsatisfactory results, presumably because the dwelling area of the ions extended beyond the homogeneous zone of the magnetic field which is normally produced by a cryomagnet. The consequences necessarily had to be distorted lineshapes and a reduced resolving power of the spectrometer. In order not to substitute a new problem for an old one, the d.c. potentials applied to the cell should, therefore, limit the dwelling area of the ions to the homogeneous zone of the magnetic field.